David Cameron has insisted the “Union is secure” despite the prospect of the SNP winning dozens of constituencies in the general election.

The Prime Minister told The Daily Telegraph that he is not concerned by the prospect of Nicola Sturgeon, the SNP leader, attempting to force another independence referendum.

He said that last year’s independence referendum was “the chance of a lifetime” and will not happen again.

It came as Mr Cameron on Tuesday visited all four nations in the UK to warn of potential “chaos” under Labour if Ed Miliband wins the general election.

David Cameron tells Ukip voters - it's time to come home

Mr Cameron has been warned that Conservative warnings about Mr Miliband being “held to ransom” by the SNP if there is a hung Parliament are bolstering the support of Miss Sturgeon’s party.

Critics have warned that it could lead to Miss Sturgeon dramatically increasing the number of SNP MPs in Westminster, which could in turn lead to demands for another independence referendum.

However, Mr Cameron indicated that the UK government would not allow another referendum.

He said: “We’ve had a referendum. Alex Salmond said it was the chance of a generation if not the chance of a lifetime and I agree with that. So I think the Union is secure.”

Asked whether a Conservative victory at the election will simply “bolster” Miss Sturgeon’s bid for independence and another referendum, Mr Cameron added: “I don’t think so. The people of Scotland know that there are two governments.

“There’s the government they elected in Holyrood, which is increasingly running larger parts of public services and other things in Scotland including their tax raising powers, and there’s a government in the United Kingdom, which is there to represent all of the United Kingdom including Scotland.”

The Prime Minister began his tour of the UK in Edinburgh, where he again warned against an potential “unholy alliance” between Labour and the SNP after the election.

David Cameron and his wife Samantha have breakfast in Edinburgh

He attacked Ms Sturgeon over her desire for Scottish independence and borrowing plans on a campaign visit to Edinburgh during a whirlwind tour of all four nations of the UK in a day.

Mr Cameron said a government led by Mr Miliband and propped up by the SNP would result in more "taxes, spending, debt and unlimited welfare".

He said: "This is the most important election in a generation. Clearly, the election here in Scotland is also crucial and we've got a very clear message, which is if you want a non-socialist alternative to the SNP there is only one choice and that's the Conservatives led by Ruth Davidson."

The Prime Minister added: "Nicola Sturgeon wants two things. She wants to break up the United Kingdom, I'm totally opposed to that, and she wants more borrowing, more spending, more taxes, more debt, more unlimited welfare, and I totally oppose that.

"I think she's wrong on the two big questions of our time, so that's why we're fighting such a vigorous campaign here, but we're also warning across the United Kingdom over the danger of this alliance potentially between the people who want to bankrupt Britain, Labour, and the people who want to break up Britain, the SNP, and pointing out the only way to avoid that is a Conservative majority government.

"I think, frankly, Ed Miliband should be clearer about what he would do with respect of the SNP, rather than at the moment he's failing to speak out because he's quite happy to ride into Downing Street on their coat-tails."

Mr Cameron then travelled to Northern Ireland, where he visited the set of fantasy television programme Game of Thrones.

David Cameron visits Game of Thrones set during Belfast trip

He then travelled to Wales and, while visiting a brewery, was forced to rule out having another child for the second time in less than a week, following comments by his wife Samantha.

Appearing at Brains brewery in Cardiff, the Prime Minister’s wife was handed a glass of Welsh Black stout and said: "I normally just drink this when I'm pregnant."

The Conservative leader interjected: “That is not an announcement!"

On Friday, Mrs Cameron held a seven-week-old baby during a visit in Lancashire, prompting the Prime Minister to say that his wife looked "broody" and joke: "Want another one?"

Afterwards he added: "Whenever I see a small baby I feel broody, but don't worry — there's nothing going to happen on that front."

A picture of the pies Sam and I baked with apprentices at Brains Brewery, Cardiff. Sam's is the neat one... pic.twitter.com/3CMN64gKBJ

Later the Prime Minister appeared in north Cornwall, which he said Mrs Cameron was keen to visit due to Poldark, the popular TV drama.

Mr Cameron jokes that he has a one pack, not a six pack, and that it causes less of a ripple when he takes his shirt off.

The Prime Minister says he will finish the day with a Doom Bar and fish and chips.

Meanwhile, Nick Clegg, the Deputy Prime Minister, attacked Mr Cameron after it emerged that Tory strategists are plotting to “destroy” the Liberal Democrats in seats across southern England and the north west.

Mr Clegg accused Mr Cameron of “disparaging talk” and said that the Prime Minister knows that he is not going to win the election outright.

Asked about the Prime Minister’s plan to target Lib Dem seats, Mr Clegg said: “Of course he would say that, he's the leader of a Conservative Party that wants to take seats off us.

"The problem for David Cameron and the Conservatives is, whatever their brave or disparaging talk, everybody knows - and they know in their heart of hearts - that they are not going to win the election."

He warned that the "swivel-eyed brigade" in the Tory party could hold sway over Mr Cameron unless a Conservative administration was held in check by the Lib Dems.

"If you have David Cameron running after Nigel Farage, or even David Cameron and Nigel Farage in Downing Street together or the Conservative Party leadership basically at the mercy of the slightly swivel-eyed brigade on their back benches," he said.

Mr Clegg also attacked Michael Gove, the former Education Secretary, criticising him for rewriting the history syllabus according to his personal whims.

Mr Gove was replaced as Education Secretary by Nicky Morgan in Mr Cameron’s final reshuffle.

The Deputy Prime Minister said that when he was in charge of the department, Mr Gove has started to act like "something out of The Thick of It" - the popular BBC political satire - and indicated he became too autocratic in the role.

Mr Clegg said: "I remember when I thought 'this is just getting absurd' was when someone explained to me that Michael Gove was personally handwriting lists of which medieval kings British schoolchildren should learn, according to his personal recollection of which kings and queens are important.

"I just thought 'this is something out of The Thick of It'. You have the Secretary of State personally instructing the hapless children of this country which medieval kings you want them to learn by rote.

"I'm afraid this happens in government, I've seen this, where a secretary of state it all slightly goes to their head, they think it's their personal fiefdom or their personal gift."

With one month until the polls open, Mr Cameron made his most emotional appeal to people who have flirted with supporting Ukip and said: “Now please, come on, let’s get together and take the country forward.” He made the comments as it emerged that the Conservatives are plotting to win the election by “decapitating” the Liberal Democrats in seats in southern England and across the North West.

Prime Minister David Cameron and Chancellor George Osborne at a conservative Party meeting held at the Bristol and Bath Science Park (Julian Simmonds/The Telegraph)